Flip-chip bonding is a well-known technology in which semiconductor dies or packages are turned upside down and attached to a substrate (e.g., a semiconductor wafer or printed circuit board (PCB)) using solder balls or joints instead of being attached in more conventional ways, such as being placed right side up and attached to a substrate using perimeter bonding wires. FIG. 1 illustrates a flip-chip bonded configuration 100, in which a die 102 is mounted to a substrate 106 via flip-chip bonding. The die 102 is suspended above the substrate 106, is affixed to the substrate by a plurality of solder balls 110, and has a lower surface 104 facing the substrate, creating a volume 105 between the surface 104 of the die and the substrate surface 108.
One common problem with flip-chip bonding occurs due to differential thermal expansion between the substrate 106 and the die 102. In many devices, the substrate 106 has a coefficient of thermal expansion that is substantially different than the coefficient of thermal expansion of the die 102. As with most semiconductor devices, when the device 100 operates it experiences a substantial increase in temperature; the temperature rise is directly related to the amount of electrical energy used by the device, which is eventually turned into heat. The differences in coefficient of thermal expansion between the die and the substrate, coupled with large temperature increases, result in substantially different amounts of thermal expansion and contraction in the substrate and the die. Since the die is rigidly connected to the substrate by the plurality of solder balls 110, the loads created by the differential thermal expansions are carried entirely by the solder balls. Because the solder balls 110 are typically very small, the result is a high stress concentration in the solder balls. These stress concentrations can result in premature failure of the solder balls and, consequently, premature failure of the entire device 100.
FIG. 2 illustrates one approach that has been used to reduce the problems caused by differential thermal expansion of the die and the substrate. In this approach, a material known as an “underfill” 202 is dispensed into the volume defined by the lower surface 104 of the die and the upper surface 108 of the substrate 106. The underfill material 202 is fairly rigid when cured, such that by filling the volume 105 with underfill material any loads arising from the differential thermal expansion of the substrate and the die, as well as any other mechanical forces that may be applied to the die, are transferred into both the solder balls and the underfill material. Since the applied loads are now carried over a substantially larger area (i.e., the area of the solder balls plus the area of the underfill material), the resulting stresses are lower and stress concentrations at the solder balls are eliminated.
Although the approach of using underfill is beneficial, it suffers from some disadvantages. Among those disadvantages is that the underfill approach is incompatible with dies including optical devices. Underfill materials are typically opaque, and therefore cannot be used in devices having optically active areas on the lower side 104 of the substrate because the underfill material would absorb any radiation radiating from or being received by an optically active area on the lower side 104 of the die, thus rendering useless the optically active area. In addition, most optical devices that would be used for the optically active area are very delicate. Since underfill materials are designed to be rigid (e.g., having a modulus of elasticity, or Young's modulus, of 7–10 GPa) so they can take up thermal and other loads, the underfill material will transfer loads to the optically active area and can damage the area.